On Wednesday, the world learned about a bug in the popular Unix, Linux, and Mac OS X command line interpreter Bash.

Discovered by engineers at Red Hat, this bug is known as Shellshock, and allows an attacker to run commands in the Bash shell. Since the bug was announced, Bash has been updated for the major platforms it affects—so it’s pretty easy to update and protect systems.

But there’s a problem: Bash is so widespread, and installed on so many devices—such as cable modems, routers, and other devices with embedded Linux operating systems—that it will be difficult, if not impossible, to fully patch everything that’s affected. (Windows users are generally unaffected by Shellshock, unless they’ve specifically installed Bash along with Cygwin, Git Bash, or other third-party packages.)

And that’s what makes it so important to update and protect what you’re able to fix.